This is because our immune system has a memory, making a note of how to fight every bug it has ever encountered.

When a bug is encountered a second time, the immune system reaches into its memory and releases weapons — called memory lymphocytes — that it knows will beat it.

So, regular nail biting exposes us to small amounts of potentially immune-boosting bugs. (The same principle applies to picking your nose and consuming the result. ‘It won’t harm you, though you’ll be a social outcast,’ she says.)

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BURPING

Don’t tell your children, but a loud burp — or belch — though offensive, may in fact protect your body against damage from stomach acid.

The sound comes from the rush of gas as it passes through the valve in the gullet at the bottom of the throat.

This valve, the upper oesophageal sphincter, then vibrates, says Dr Nick Read, a consultant gastroenterologist for the charity the IBS Network.

Burp gas is formed of a mixture of substances. As well as containing air we swallow when we bolt down food, it also contains carbon dioxide.

This is produced in the stomach when the acid mixes with alkaline bile (which comes from the section of gut below the stomach).

Certain fat-rich food, such as chips or creamy sauces, alcohol and smoking can exaggerate this process, called duodeno-gastric reflux.

This natural gas release — the belch — is a normal part of digestion and suppressing it can cause problems.

‘If you don’t belch and the gas stays on the stomach, this can cause the valve that separates the gullet and the stomach to relax, allowing stomach acid to splash up into the gullet, triggering heartburn,’ says Dr Read.

It can also cause the lower end of the gullet to go into spasm, he says, triggering pain in the centre of the chest.

Though belching is a normal part of digestion, certain conditions can cause someone to burp more than usual.

These include stomach ulcers or inflammation of the duodenum or gullet, which can be caused by aspirin or alcohol.

There are also conditions that can produce more unpleasant belches.

‘If no stomach acid is being produced — which can be due to an infection with the helicobacter pylori bug or the condition pernicious anaemia (where the body doesn’t absorb enough vitamin B12) — then this changes the type of bacteria in the stomach,’ says Dr Read.

In some cases, these bacteria produce hydrogen, which can make belches flammable.

‘One patient told me she lit up a cigarette and two plumes of flames came out of her nostrils,’ says Dr Read.

PASSING WIND

As with burping, it’s important that we pass wind.

‘We evacuate wind for a reason — it forms in the bowel and we need to get rid of it,’ say Dr Read.

Most of the gas comes from the fermentation of protein and carbohydrate. Gas is usually produced from your bottom around six hours after eating.

‘If you eat at 7pm, by 2am you’ll feel it bubbling away in your lower abdomen and may start to produce gas,’ he says.

‘Your may even feel your intestines, specifically our caecum (the first section of the large intestine), start to expand in the lower right-hand corner of your abdomen.’

Releasing the gas eases pain and bloating, especially if you have a sensitive gut that becomes bloated regularly.

‘Holding it back can also trigger pain. A colleague used to call it Metropolitan Railway Syndrome — all these commuters suffered pain and bloating because they were too embarrassed to break wind on public transport.’

KNUCKLE CRACKING

Knuckle cracking may make our joints feel more flexible

The loud pop of someone cracking their knuckles makes most people wince, but though it sounds harmful it has no effect on the health of our joints — and may make the joint feel more flexible.

‘There is a lot of folklore surrounding this, with some people claiming it weakens the joints,’ says Dr Chris Edwards, consultant rheumatologist at Southampton General Hospital. ‘But when you look at the evidence it doesn’t seem to have an effect.

‘One large study followed a group of people for five years and looked at whether those who cracked their knuckles were more likely to develop arthritis — they found their joints were just as healthy as those who didn’t.’

The sound is thought to come from the air pockets that form in the liquid — the synovial fluid — that surrounds each joint; the crack is produced when these pop under the pressure of the joint bending.

‘This cracking can make a joint feel more flexible,’ says Dr Edwards.

‘People’s joints tend to feel more comfortable after cracking — this may be because they have stretched out the joint and have a greater degree of movement.’

EATING IN BED

Eating in bed: Make sure you sit upright and prop up yourself with pillows, because hunching can squash the stomach and trigger indigestion

Despite what your mother told you, eating in bed may aid digestion.

This is because digestion relies on the parasympathetic nervous system being activated, specifically a long nerve called the vagus nerve.

Stress of any sort will interfere with this mechanism and try to stop digestion.

‘If you’re eating in a rush, are stressed or tense, you’ll get a conflict in the gut and end up with indigestion or other gastric symptoms, such as bloating,’ says Dr Read.

He adds that the best way to eat your meal is in a relaxed environment where you can take time to enjoy your meal. And what better place to do this than in your own bed?

‘Though it’s always best to eat at a table with friends or family, at least in bed you are relaxed,’ says Dr Read.

Make sure you sit upright and prop up yourself with pillows, because hunching can squash the stomach and trigger indigestion.

SPITTING

Spitting may be a filthy habit, but when you’re exercising it could help you breathe more easily.

When we exercise, our mouth and throat produce more saliva, says Dr John Dickinson, lecturer in exercise physiology at the University of Kent.

‘Normally, we breathe through our nose — this warms the air and makes it more humid, allowing the body to absorb oxygen from it more efficiently.

‘However, when we exercise we tend to breathe deeply through our mouth to draw as much air as possible into the body.

‘But this air isn’t being warmed up or humidified, and when this cold, dry air hits the back of the throat, the cells try to protect themselves by triggering inflammation.’

Though not harmful, this inflammation cause the cells lining the throat to produce a layer of saliva that shields them from the cold air.

Dr Dickinson says runners may find the saliva can build up and interfere with their breathing pattern, so they need to spit.

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Bad habits: From biting your nails to burping and even eating in bed: The bad habits that can be GOOD for you!